1. Field of the Invention
The invention pertains to the field of tensioners. More particularly, the invention pertains to a chain or belt tensioner with a ratchet that deactivates.
2. Description of Related Art
In chain and belt tensioners, pistons are urged from their housings by a spring or a spring plus hydraulic pressure. Check valves are added to tensioners between the pressurized fluid source and the fluid chamber formed in the bore between the housing and the piston, to prevent backflow of the hydraulic fluid out of the fluid chamber. The rate of leakage through the clearance between the piston and the bore allows the retraction of the piston as makeup fluid subsides or ceases. As the rate of leakage increases, the resistance to retraction of the piston lessens and as the rate of leakage decreases, the resistance to retraction of the piston increases. When chain load spikes or a loss of fluid pressure is present, retraction of the plunger occurs and if the retraction of the plunger is excessive, loss of chain control, loss of engine time or other undesirable effects occur. Therefore, limiting the amount of piston retraction is desirable.
The common practice of tuning a tensioner to a system by changing the fluid leak rate occasionally results in a desired normal state piston retraction, which exceeds what is desired during startup or engine operation and shutdown. Tuning a tensioner to properly control system dynamics may also result in excessive piston extension or pump-up that may cause high chain loading or piston binding. This often results in a tensioner tune that is less than optimal in order to prevent the piston from extending beyond the allowable limit.